A detailed characterisation of the influent and the effluent wastewater for one year period and a performance evaluation was carried out for the wastewater treatment plant of Cumhuriyet University (Sivas, Turkey). The objective of this work was to investigate the performance of different units in overall treatment of the plant and particular attention was given to the performance of the extended aeration reactor based on different operational parameters. Some treatment units were found to be ineffective in the overall treatment performance. A new process flow suitable for this treatment plant was developed by constructing a by-pass channel. As a consequence, the plant was upgraded to provide adequate treatment efficiencies with lower operational costs.